This invention relates to protective coating systems for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a method of inhibiting the formation of deleterious topologically-close packed (TCP) phases in a superalloy protected by an aluminum-rich coating by nitriding the superalloy surface before depositing the aluminum-rich coating.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, these components are often protected by an environmental and/or thermal-insulating coating, the latter of which is termed a thermal barrier coating (TBC) system. TBC systems typically include an environmentally-protective bond coat and a thermal-insulating ceramic topcoat, typically referred to as the TBC.
To be effective, TBC systems must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials typically used to form turbine engine components. TBC systems capable of satisfying the above requirements typically employ a bond coat formed of an oxidation-resistant aluminum-containing alloy such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), or an oxidation-resistant diffusion coating, such as diffusion aluminide coatings that contain aluminum intermetallics. These same compositions are often used alone as environmental coatings for superalloy components that do not require the added thermal protection of a TBC.
When bond coats and environmental coatings of the type described above are applied, a zone of chemical mixing occurs to some degree between the coating and the superalloy substrate. This zone is typically referred to as a diffusion zone (DZ), and results from the interdiffusion between the coating and substrate. For many alloys, it is typical to see topologically close-packed (TCP) phases in the diffusion zone after high temperature exposures, e.g., near or above about 900xc2x0 C. The incidence of a moderate amount of TCP phases beneath the coating is typically not detrimental. However, certain high strength superalloys contain significant amounts of refractory elements, such as tungsten, rhenium, tantalum, molybdenum and chromium, which are all components of TCP phases. If these elements, and particularly rhenium, are present in sufficient amounts or combinations, a particularly detrimental type of diffusion zone containing deleterious TCP phases can form after coating. This instability was first seen beneath the diffusion zone of an aluminide coating, and has been termed a secondary reaction zone (SRZ). SRZ and/or its boundaries readily crack under stress and remove useful load-bearing area through its growth into the superalloy substrate.
Commonly-assigned U.S. Pat. No. 5,334,263 to Schaeffer teaches a method of inhibiting the formation of deleterious TCP phases in a superalloy protected by an aluminum-rich coating by carburizing the superalloy surface before depositing the aluminum-rich coating. According to Schaeffer, carbon can be diffused into a superalloy substrate to tie up certain TCP phase-forming refractory elements, and to serve as a barrier between the subsequently-deposited aluminide coating and the superalloy substrate to prevent interaction between the two. However, a limitation to this approach is that only those refractory elements that will form a stable carbide are affected. As a result, an element such as aluminum, which is not a carbide former but important in the occurrence of TCP formation, is not affected by carburization. However, aluminum content is critically related to alloy stability. Increasing the aluminum content in an alloy increases its gamma prime amount. Since refractory elements such as molybdenum, chromium, rhenium and tungsten do not normally partition to the gamma prime phase, their concentration in the remaining gamma phase is increased, producing a higher electron vacancy number and an increased propensity to form detrimental TCP phases.
The present invention generally provides a coating system and method for forming the coating system on an article designed for use in a hostile environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The method is particularly directed to inhibiting the formation of deleterious topologically-close packed (TCP) phases in a superalloy protected by an aluminum-rich coating and optionally a thermal insulating ceramic layer. Superalloys of particular interest are those containing significant levels of TCP phase-forming elements, such as tungsten, rhenium, tantalum, molybdenum and chromium.
According to this invention, the formation of deleterious TCP phases in the near-surface region of a superalloy substrate can be inhibited by nitriding the substrate prior to depositing the aluminum-rich coating. Within the nitrided surface region, nitrides of the elements of concern will be present, such as aluminum, tantalum and chromium. Titanium, boron, zirconium and niobium nitrides may also be formed if these elements are present in the base alloy. These nitrides are not detrimental to the physical, mechanical and environmental properties of the superalloy if limited to about 10 volume percent within the near-surface region and 20 micrometers in size.
Following nitriding, an aluminum-rich coating can be deposited on the nitrided surface region, yielding an aluminum-rich diffusion zone that extends into the nitrided surface region from the aluminum-rich coating. In a preferred embodiment, the diffusion zone extends into but not beyond the nitrided surface region. During deposition of the coating, less stable nitrides can dissolve and be replaced with more stable nitrides, such that the formation of aluminum nitride continues.
According to the above, appropriately nitriding the surface of a superalloy component serves to form stable nitrides that tie up more TCP phase-forming elements present in the near-surface region of the superalloy than possible by carburizing. By reducing the incidence of detrimental TCP phases, the service life of a superalloy component can be considerably improved, particularly if the superalloy contains relatively high levels, e.g., 5 weight percent or more, of detrimental TCP phase-forming elements such as of aluminum, rhenium, tungsten and/or tantalum.
Other objects and advantages of this invention will be better appreciated from the following detailed description.